Aerosol Workshop — June 2-3, 1997

Session 5:
Links Among Satellites, Models, In Situ and Surface Data

(Facilitator: John Ogren; Recorder:
Ralph Kahn)

Aerosol Transport Models +
Satellite Data

Steve Schwartz, Brookhaven National
Laboratory

Published chemical transport models for aerosols or
for specific aerosol species such as sulfate,
especially those that are driven by monthly mean winds
from climatology, exhibit smooth contours. The
repeated publication of figures showing such smooth
contours may have prejudiced thinking that this is in
some way representative of the actual distribution of
aerosol loadings. Seasonal composites of
satellite-derived aerosol optical depth likewise
exhibit rather smooth contours. Reality is very much
otherwise. The short residence times of tropospheric
aerosols are comparable to the time scale of
variability in synoptic scale winds and precipitation
that control the distribution of aerosols. This
situation, together with the highly nonuniform
distribution of sources of anthropogenic and dust
aerosols, leads to a highly heterogeneous distribution
of aerosol loadings (in great contrast to the rather
smooth distribution of the long-lived greenhouse
gases). The heterogeneity in aerosol loadings is
readily manifest in time series of aerosol loadings at
a given location. Likewise one should expect a
similar variation in the spatial distribution, as is
exhibited in models for which the controlling
meteorology exhibits the short-time variability of
actual synoptic scale variability, but the synoptic
observational coverage required to develop this
picture is lacking (one might imagine trying to get
this from GOES, although the photometric resolution is
marginal). Models can be highly valuable in trying to
discern the aerosol loading in observations, provided
they accurately represent the temporal heterogeneity
that is responsible for the heterogeneity in loadings.
For this it is necessary to drive the model by
observation-derived meteorological data, not by
climatological or GCM meteorology. This situation may
be turned to advantage, because the inherent spatial
and temporal variability of aerosol loading allows the
possibility of experiments that can discern and
quantify the aerosol influences, by comparisons
between high and low loading situations.

Three-Dimensional Global
Modeling with Cloud-Aerosol Physics

Steve Ghan, Pacific Northwest
Laboratories

To facilitate comparison between simulated and
observed aerosol, all aerosol models should be either
driven by observed meteorology or by a GCM that is
nudged toward observed meteorology.

Aerosol radiance (the difference between the true
radiance and the radiance if aerosols were not
present) should be used to evaluate aerosol models.
Use of aerosol optical depth is not as closely related
to aerosol radiative forcing and introduces
uncertainties in the "observations".

There does not appear to be any way to determine
cloud droplet number concentration from current or
planned satellite measurements. The combination of a
mm cloud radar and a microwave radiometer on a
satellite would fulfill this critical need for
estimating indirect radiative forcing.

Pinatubo Radiative Forcing
and Climate Response

Alan Robock, University of
Maryland

Scientists use a variety of definitions of
"Radiative Forcing." What the climate system actually
feels is in situ radiative heating of the atmosphere
and changes of net shortwave and downward longwave
flux at the surface. All radiative forcings should be
evaluated in these terms to compare them. This can be
illustrated by looking at the effects of Pinatubo
aerosols on visible, near-IR, and thermal IR in both
the troposphere and stratosphere.

Stenchikov et al. (Max Planck Inst. fur Meteorol.,
Rept. 231, Hamburg, 40 pp., 1997; submitted to
J. Geophys. Res., March 1997) developed a
detailed data set of stratospheric aerosol properties
and distribution for the 2-year period following the
1991 Pinatubo eruption and produced heating rates and
fluxes from the data, using the Max Planck Institute
ECHAM-4 GCM. We suggest that other GCMs use these
same data sets (aerosols, or heating rates and fluxes)
as forcing, to compare the radiation schemes and
climate response. Stratospheric aerosols should be an
important part of the NRA, as they offer important
episodic (event-based) forcings which are important
for the climate system.

Factors Governing Aerosol
Radiative Forcing

V. Ramaswamy, NOAA Geophysical
Fluid Dynamics Laboratory

A principal factor affecting aerosol radiative
forcing is the dependence on humidity. Thus far, most
if not all estimates of anthropogenic aerosol forcing
have been made using general circulation model (GCM)
climatologies. Since sub-grid scale variation of
humidity is a major uncertainty in GCMs, and as
aerosol hygroscopicity and the accompanying growth are
nonlinear functions of humidity (especially at
relative humidities greater than 85%), this becomes an
important element to consider in assessing the
accuracy of the aerosol radiative forcing. Haywood et
al. (GRL, 24, 143-146, 1997)
demonstrate that the impact of considering a fine
spatial resolution of the humidity field versus a
coarse one can lead to significant biases in the
forcing estimates. It is, hence, a factor to be kept
in mind when attempting to verify model estimates with
observations.

GCM simulations of anthropogenic aerosol
concentrations, together with a model climatology of
humidities and cloud distributions, have been used to
investigate the sensitivity of the (negative)
hygroscopic sulfate and (positive) hydrophobic soot
aerosol forcings (Haywood and Ramaswamy,
JGR, submitted, 1997). It is found that
different model simulations of the anthropogenic
sulfate aerosols differ considerably in the vertical
distribution despite similar column burdens. This
arises due to differences in the horizontal and
vertical transport of species in the various models.
These differences result in varying estimates of the
radiative forcing. The sulfate aerosol forcing, being
largely governed by the relative humidity profile, is
greater when the major portion of the columnar mass is
located lower in the troposphere. In contrast, black
carbon yields a greater forcing when a substantial
mass is present above low clouds; then, the multiple
scattering process leads to an enhancement of the
absorption.

Integrating Aircraft
Measurements, Satellites and Models

Antony Clarke, University of
Hawaii

Assessing global radiative forcing and related
climate issues depends upon analysis of validated
satellite data. One approach to validation is through
comparison to calibrated surface measurements (e.g.,
AERONET), but by itself such a comparison has many
ambiguities. Effective linking and interpretation of
satellite observations and model simulations require
knowledge of the vertical characteristics of the
aerosols, implying the need for aircraft and/or lidar
measurements. A strategy for integrating limited
resources to best support a satellite/model validation
and interpretation effort should consider the use of
light 2-4 seat aircraft or remotely piloted vehicles
(RPVs), which offer cheap, flexible and frequent
sampling opportunities, as well as the use of
traditional major aircraft campaigns.

Aircraft data need to be linked appropriately to
both satellite and modeling needs. Given aerosol
variability, appropriate spatial and temporal scales
must be established better, as well as the most
uncertain or sensitive parameters needed by the
modeling community. Lidar measurements, which in the
past decade have revealed complex layering of
aerosols, can be especially useful for placing in situ
measurements in context, thus helping to link in situ
data to models and satellite retrievals. These
considerations lead to a suggested strategy for
enhancing the integration of models, satellite data,
and other measurements [see viewgraph of Clarke in
Panel C (Strategy)].

Aircraft data must play a role, in combination with
models and satellite data, in analysis of the
challenging indirect aerosol forcing issue. A large
number of factors influence cloud albedo and lifetime,
in addition to possible effects of aerosol
perturbations, which thus complicates interpretation
of even well-conceived field experiments. Anomalous
aerosol conditions, as exist in large well-defined
plumes (such as the volcanic plume from Kilauea,
Hawaii or possibly an isolated city plume over
pristine ocean) can provide exaggerated conditions to
study indirect effects on clouds. It would be useful
to find situations in which there is an extended
persistent gradient in aerosol properties in a region
with reasonably uniform cloud fields, dynamics and
surface temperature. Interpretation will probably
depend upon having satellite data over many years to
verify the existence of statistically robust cloud
effects, consistent with the notion of "events in
global decadal context".

Regional Modeling with Data
Assimilation

Doug Westphal, Naval Research
Laboratory

I suggest that we evaluate our ability to model
aerosols (sources, transport, sinks) by applying
models (global or regional) to observed events.
Questions to be answered using this approach
include:

What resolution (spatial or temporal) is required
to accurately model sources, the near-source nonlinear
microphysics and chemistry, and sinks (precipitation).

Does the dynamical model capture the
characteristics of the meteorological forcing,
including RH, PBL depth, precipitation, easterly
waves, etc.?

How can we combine data from disparate satellite
retrievals into a single product? What is the
accuracy?

Session 5 Summary by
Recorder(Ralph Kahn)

John Ogren(as Facilitator).
Regarding the link between models and observations:
(1) Observations provide estimates of aerosol
properties, their variability and uncertainties. (2)
Models provide sensitivity of climate to aerosol
properties. So the two communities must interact.

Steve Schwartz. Aerosol properties and
amounts vary on the space and time scales of synoptic
meteorology, which governs aerosol transports.
"Averaged" values, (e.g., smoothed contours) eliminate
the peaks and valleys, and are inadequate to represent
aerosol non-linear effects, such as those involved in
aerosol interactions with clouds. Both models and
observations must capture aerosol variability on
synoptic scales.

Steve Ghan. Satellite observations of
radiance are usually converted to aerosol properties,
which are used in a model to generate radiative
forcing. This procedure involves a set of assumptions
that can be avoided if model-produced radiances are
compared directly to satellite-derived radiances.

Ghan also emphasized the variability of aerosols on
synoptic scales.

Alan Robock. Robock recommends using the
same forcing in multiple models to explore model
behavior. Stenchikov and Robock developed a detailed
data set of stratospheric aerosol properties and
distribution for the 2-year period following the 1991
Pinatubo eruption. They produced heating rates and
fluxes from the data, using the Max Planck Institute
ECHAM-4 GCM. They suggest that other GCMs use these
same data sets (aerosols, or heating rates and fluxes)
as forcing, to allow comparison of the radiation
schemes and climate response.

V. Ramaswamy. The vertical distribution
of aerosols matters, e.g., for hygroscopic aerosols in
a model where relative humidity varies with height, or
for absorbing aerosols when there is cloud in the
atmospheric column. The horizontal distribution of
aerosols matters, e.g., for hygroscopic aerosols in a
situation where relative humidity varies horizontally.

Ramaswamy also pointed out the danger of
representing sub-grid-scale variability with
"averaged" values, because of the non-linear
dependence of aerosol properties on relative humidity
when RH >~ 80%.

Clarke also pointed out variability of aerosol
properties on synoptic spatial and temporal scales.
There are some relatively inexpensive aircraft
packages to measure aerosol properties in situ. These
can be used to identify the cases when surface aerosol
properties are or are not representative of those in
the entire column.

Doug Westphal. It is important to resolve
aerosol sources and sinks at the correct scales. For
example, you need the detailed, local wind pattern to
understand dust sources in the Gulf of Oman region.
Weekly "averaged" AVHRR aerosol retrievals show a dust
"plume" off West Africa whereas the dust is actually
transported in eddies mixed with clouds.

Westphal recommends an "events-based strategy" in
which attention is paid to the scales necessary to
resolve key attributes of the event, such as looking
at "bright spot" dust sources over deserts in the TOMS
daily data.

Bill Rossow. Pointed out that
event-specific data handling operations may be helpful
in characterizing phenomena of interest in the
observations, such as studying all the easterly-phase
waves off West Africa for the Sahara dust transport
over the Atlantic.